Over recent years, there has been a trend towards processing audio signals in the digital domain, in which an analogue input signal is digitized by an analogue to digital converter, processed (possibly in a digital signal processing environment) and then converted back from a digital signal in to an analogue output signal. However, in parallel with this trend, demand also exists for purely analogue processing systems in which audio signals continue to be represented as analogue voltages and analogue processing is performed upon these voltage signals.
A characteristic of early amplification systems is that they tend to introduce a degree of distortion. Distortion is brought about by the existence of non-linear elements and processes within the system therefore the presence of these non-linearities, when considered in the frequency domain, results in the introduction of new frequency components, these being harmonics of the original component frequencies.
Poor quality bipolar transistor amplifiers have a tendency to introduce third harmonic distortion, also referred to as symmetrical distortion. The resulting harmonics are not musically related to the originating frequencies therefore the distortion is perceived as being unpleasant. However, some types of amplification system, including early vacuum tube based amplification systems, introduce even harmonic distortion in which the resulting harmonics are musically related to the originating frequencies. Under these circumstances the resulting distortion is not unpleasant and some musicians prefer the introduction of this type of distortion, as they consider the undistorted amplification process to be less tasteful and somewhat “cold”.
Given the potential desirability of even harmonic distortion, particularly in analogue systems, attempts have been made to recreate the distortion so that the presence of the distortion and the degree of the distortion may be selected in accordance with a performer's taste.
Known procedures for introducing distortion through an amplification process often involve overloading the input stages of the amplifier. Thus, distortion may be introduced by increasing the level of the input signal. Consequently, input stages of the amplification process are overloaded, introducing a desired distortion, whereafter the distorted signal is further amplified to produce the required output. Such an approach is adopted in many guitar amplification systems.
A problem with the amplification of signals originating from microphones and similar audio transducers is that the level of the input signal can vary significantly compared to, say, output signals generated by electric guitar pickups. For example, a microphone placed near to a drum will produce a relatively high signal whereas a microphone situated in a room to collected ambient sounds will produce a very low signal. A problem therefore exists in terms of introducing variable levels of distortion without being dependent upon the level of the input signal.